In general, video data is compressed to be stored or transmitted due to its large data size and the compressed video data is decoded so as to be reproduced.
FIG. 1 is a diagram illustrating a conventional video compression method.
Referring to FIG. 1, video compression is performed in units of still images included in video data.
In terms of a compression method, an original input image may be compressed by using a P/B frame method in which temporal redundancy is removed by motion prediction and motion compensation using a reference image, or an I frame method in which the image is directly compressed not by using the motion vectors.
In block-based video compression, an image is divided into macroblocks and sub-blocks and then is compressed by performing motion prediction in block units.
In the I frame method, the original image is compressed by discrete cosine transform (DCT), quantization and entropy encoding and is stored into a buffer by inverse quantization and inverse discrete cosine transform (IDCT) so as to be used as a reference image when a temporal redundancy of P and B frames is removed by motion prediction.
In the P/B frame method, the motion prediction and the motion compensation are performed on the original input image using the stored reference image and a difference image between the original image and the reference image is obtained. Then, the difference image is compressed by DCT, quantization, and entropy encoding and motion vectors are encoded.
The above-described image compressed by using the P/B frame method may be stored as a reference image for an image to be input later by inverse quantization and IDCT and by being combined with a motion-compensated reference image.
For the motion prediction and the motion compensation, one reference image is used in a P frame method and two reference images are used in a B frame method.
FIG. 2 is a diagram illustrating a method of reproducing a bitstream compressed by the method illustrated in FIG. 1.
Text information and motion vectors having losses are input. An I frame is reproduced after being entropy-decoded, inverse quantized and IDCT-converted, and the reproduced image is used as a reference image.
A P or B frame is reproduced by being entropy decoded, inverse quantized, and IDCT-converted, and then motion-compensated by using input motion vectors and the reference image. The reproduced image may be used as a reference image.
Conventional H.264 video coding standards provide a switching picture technology which efficiently allows a method of lossless encoding of a decoded image. The technology includes an encoding technology for a non-switching picture, a SI picture, and a switching picture. One non-switching picture included in a bitstream may be lossless-encoded and switched by a SI picture or a switching picture.
FIG. 3 is a diagram for illustrating a disadvantage of switching picture bitstreams in a conventional H.264 video coding technology.
Video data is compressed into a high quality video bitstream and a low quality video bitstream in IPPPP structure by separately using a video compressor. A user is watching image Pl5 of the low quality video bitstream being reproduced at a predetermined time point.
In order for the user to continuously watch the video data while changing from low quality video bitstream to high quality video bitstream in accordance with the wishes of the user, the user has to reproduce next image Ph6 in the high quality video bitstream.
However, in order to reproduce image Ph6, reference image Ph5 has to be reproduced and thus, the high quality video bitstream has to be decoded from Ih1.
FIG. 4 is a diagram for illustrating a disadvantage of switching picture bitstreams in a scalable video coding (SVC) technology.
A video data is compressed into an enhancement layer video bitstream and a base layer video bitstream in IPPPP structure by using a new different video compressor.
Here, P is compressed by using, for example, an interlayer motion prediction and an interlayer texture prediction used for a new video compressing technology.
A user is watching image Pl5 of the base layer video bitstream being reproduced at a predetermined time point.
In order for the user to continuously watch the video data while changing from low quality video bitstream to high quality video bitstream in accordance with the wishes of the user, the user has to reproduce next image Ph6 in the enhancement layer video bitstream. However, in order to reproduce image Ph6, reference image Ph5 has to be reproduced and thus, the high quality video bitstream has to be decoded from Ih1.
The above described new encoding technology can provide a bitstream switching picture by encoding the bitstream switching picture into an I picture. However, an encoding efficiency is greatly reduced.
As described above, the new video coding technology does not provide an efficient bitstream switching function. Also, other advantages provided by a switching picture technology such as a random access, error durability, and a bitstream splicing are not provided.